In recent years a need has arisen for coating compositions that will function to replace chromates in metal treatment. This is due to the detrimental health and environmental impact that has been determined to be associated with chromium compounds. There is also a need for a coating that is formed from an aqueous solution with no organic solvents used. This eliminates the disposal and emission considerations involved in producing zirconates and other metal oxide-containing coatings from sol-gel applications, while providing a broad-spectrum replacement for chromates.
There are believed to be several mechanisms by which chromates provide protection to an underlying substrate. While the complete source of the protection has not been fully elucidated, there has been considerable research to identify each aspect of the chromate mechanistic model. In Corrosion Science, 34 (1), 41 (1993), Kendig, Davenport and Isaacs used XANES to demonstrate variable valence states of chromium in chromate coatings. This revealed both the +3 and +6 oxidation states. The chromium in both states is present as oxides. The +3 state forms a stable "long-range" oxypolymer and the chromium remaining in the +6 state, which is trapped in the film, has limited long-range structure.
The protection would then come from at least two mechanistic aspects. One is the physical aspect of protection provided by the stable +3 oxide matrix. A secondary protective source is the +6 chromate in the film. The trapped reservoir of +6 chromate is in some way available to heal the film in some fashion once corrosive attack begins. Many chromate-free chemical conversion coatings for metal surfaces are known to the art. These are designed to render a metal surface "passive" (or less "reactive" in a corrosive environment), leaving the underlying metal protected from the environment. Coatings of this type that produce a corrosion resistant outer layer on the base metal or its oxide often simultaneously produce a surface with improved paint adhesion. Conversion coatings may be applied by a no-rinse process, in which the substrate surface is treated by dipping, spraying, or roll coating. The coatings may also be applied in one or more stages that are subsequently rinsed with water to remove undesirable contaminants.
Several metal and metaloid elements will form a continuous three-dimensional polymeric metal- or metaloid-oxide matrix from aqueous solutions. Chromium shares this characteristic along with silicon and other elements. The Group IV-A elements continue to be attractive candidates for chromate replacement technologies as they share the virtue of being relatively innocuous environmentally and have common valences of +4, facilitating the formation of three dimensional amorphous coatings.
Chromate-free conversion coatings are generally based on chemical mixtures that in some fashion will react with the substrate surface and bind to it to form protective layers. The layer or layers may yield protection through galvanic effects or through simply providing a physical barrier to the surrounding environment.
Many of these conversion coatings have been based on Group IV-A metals such as titanium, zirconium and hafnium, a source of fluoride and a mineral acid for pH adjustment. Fluoride has typically been considered to be necessary to maintain the Group IV-A and other metals in solution as a complex fluorides. The fluoride may also serve to keep dissolved substrate metal ions (such as aluminum) in solution.
For example, U.S. Pat. No. 4,338,140 to Reghi discloses a coating for improved corrosion resistance with solutions containing zirconium, fluoride and tannin compounds at pH values from 1.5 to 3.5. Optionally, the coating may contain phosphate ions.
U.S. Pat. No. 4,470,853 to Das is related to a coating composition comprised of zirconium, fluoride, tannin, phosphate, and zinc in the pH range of 2.3 to 2.95. According to Das, it is important that approximately 10 atomic percent of zirconium-zirconium oxide be present in the coating to obtain "TR-4" corrosion resistance. It was shown that coatings of higher zirconium oxide content produced excellent corrosion resistance. Compositions which gave higher zirconium oxide on the surface were preferred in the disclosures.
U.S. Pat. No. 4,462,842 to Uchiyama and U.S. Pat. No. 5,380,374 to Tomlinson disclose zirconium treatments in solutions containing fluorides which are followed by treatment with silicate solutions. This combination is suggested to form zirconate and syloxyl linkages (--O--Zr--O--Si--O--Si-- . . . ), yielding a coating with improved corrosion resistance over the zirconium treatment alone. Coatings of this type give excellent corrosion protection but very poor paint adhesion.
The compositions and processes of Uchiyama are useful in producing hydrophilic surfaces. The compositions of Tomlinson purportedly do the same when subsequently treated per Uchiyama. The compositions of Tomlinson are high in Group II-A metals, which somewhat improve the latent corrosion protection of the fluoro-Group IV-A coating formed. The drawback is that the solubility of Group II-A components is limited, therefore the opportunity to formulate stable concentrates may not be possible.
Additionally, coating compositions high in the Group II-A elements tend to generate considerable scaling as described by Reghi in U.S. Pat. No. 4,338,140. While an incremental improvement in paint adhesion may be afforded by Group II-A metal inclusion in some aspect of the present invention, they may actually inhibit formation of the continuous amorphous metal oxide matrices in some cases.
In Reghi and in U.S. Pat. Nos. 5,380,374 and 5,441,580 to Tomlinson, Group I-A and Group II-A elements probably incorporate as "discrete," non-bonded cations, perhaps providing some space-charge stabilization to balance discrete anions in the coatings. But these compositions likely provide little if any long-range structure.
U.S. Pat. No. 4,863,706 to Wada discloses a process for producing sols and gels of zirconium and a process for producing zirconia. The processes described include reactions to produce basic boratozirconium and basic boratozirconium chloride sols. These were purportedly used in producing boratozirconium and boratozirconium chloride gels. Further described is a method for producing zirconia from the gels at relatively low temperature. The essential components include a boron compound along with a polyvalent metal, zirconium and chloride.
U.S. Pat. No. 5,397,390 to Gorecki discloses an adhesion promoting rinse containing zirconium in combination with one or more organosilanes and fluoride. The compositions are used to rinse surfaces after they have been treated in a phosphating bath. The zirconium ion concentration is selected to maintain pH in a broad range as the silanes deposit on the substrate to promote paint adhesion and improve corrosion resistance. Organosilanes are necessary components of the disclosed compositions. Additionally, in preparing the compositions, Gorecki indicates that whenever zirconium-containing salts such as zirconium basic carbonate, zirconium hydroxychloride and zirconium oxychloride are used as a source (of zirconium) the salts must be dissolved in 50% hydrofluoric acid in order to effect dissolution. Gorecki does not indicate a necessity to dissolve the fluorozirconate salts mentioned in his disclosure. This demonstrates that fluoride is a necessary component of the disclosed compositions as it is included as part of the fluorozirconate salts or from hydrofluoric acid.
Brit. Pat. 1,504,494 to Matsushima describes a process for treating metal surfaces using zirconium at a pH above 10.0. A zirconate coating is formed but the pH of the solution is maintained above the present invention.
In pending U.S. patent application by Tomlinson, compositions with fluoride to Group IV-A metal at a molar ratio of less than or equal to two to one and zero to one are disclosed. Although the compositions are effective in providing corrosion resistance to many substrates (such as, for example, low copper aluminum alloys such as type 3003) protection from pitting corrosion on high-copper 2024 aluminum is still only on a par (depending on the activator and conditioners used) to what is seen for most chromates.
One avenue of research into protecting the copper bearing aluminum alloys has been to provide compositions that contain azole derivatives to complex any copper that dissolves during corrosive attack. This can happen through various cells that can be established at copper inclusions at the surface of these alloys. U.S. Pat. No. 5,128,065 to Hollander discloses this type of chemistry. The azoles of this type and some of those disclosed by Cha in U.S. Pat. No. 5,156,769 show some promise.
U.S. Pat. No. 3,969,152 to Mclotik discloses compositions containing rare-earth metals for improving corrosion resistance and paint adhesion to treated substrates. Generally, cerium was used in acidic systems. The aqueous compositions were effective at concentrations of 0.000001 molar and higher. Several acid-soluble transition metals were also used in the compositions and were particularly effective for sealing phosphated metals. No reference is made to the use of Group IV-A metals in low-fluoride compositions.
U.S. Pat. No. 4,359,347 to Da Fonte discloses compositions containing oxidizing agents, iron and cobalt. Additionally, cerium may be added to "activate" the bath, which, unless otherwise stated, is not analogous to the "activation language" used to describe surface activation herein. Group IV-A metals are not required in the compositions.
U.S. Pat. No. 5,192,374 to Kindler discloses compositions that form hydroxides and oxides in the pores of boehmited aluminum. The compositions were shown to be effective in improving properties of the aluminum hydrate formed by exposure to high temperature water. The present invention will also improve this type of layer with much less severe processing conditions being required to obtain the desired properties. The disclosure does demonstrate that a subsequent treatment with silicate may be useful as a seal. While benefits of adding metals such as lithium, aluminum and sodium are claimed, the Group IV-A metals are not shown.
U.S. Pat. No. 5,194,138 to Mansfeld claims non-halide compositions containing cerium, which would exclude fluoride. Although there are purported benefits to using a molybdenum treatment, the resulting systems will suffer from the absence of the amorphous polymeric Group IV-A matrix, which provides a stable physical barrier.
U.S. Pat. No. 5,209,788 to McMillen discloses compositions containing various amino compounds used in conjunction with either Group III-A or Group IV-A transition metal compounds. Group IV-A metals combined with the amino compounds and various organics are claimed for passivating phosphated surfaces. Although McMillen discloses use of the individual groups, there is no suggestion of the two metal groups exhibiting a synergistic complement with respect to each other.
U.S. Pat. No. 5,322,560 to DePue discloses combinations of rare-earth metals with Group IV-A metals in a matrix which produces a "slightly water-soluble" (no more than 0.001 moles per liter) time release corrosion inhibitor. The compositions are alkaline and applied to aluminum flake pigments. It appears the components are mixed in such a fashion that they react with each other along with additional ingredients, notably silicon salts such as sodium metasilicate. Combination of Group III-A with Group IV-A metals in an acidic, low fluoride, aqueous medium where they enjoy a high level solubility is not evident. The coatings produced by these alkaline compositions appear to be sol-gel type analogues.
U.S. Pat. No. 5,525,560 to Yamazaki discloses compositions to produce stabilized zirconia ceramics using Group III-A oxides. Temperatures for producing zirconia-based Group III-A stabilized ceramics are typically above 500.degree. C.
U.S. Pat. No. 5,362,335 to Rungta discloses cerium incorporation into an aluminum oxide surface. This is similar to use of various metals, such as nickel and certain Group III-A metals, for sealing anodized aluminum. The low fluoride Group IV-A coatings of U.S. patent application Ser. No. 08/723,464 are also effective in sealing anodized aluminum.
U.S. Pat. No. 5,399,210 to Miller discloses the combination of cerium chloride with potassium permanganate alone or in combination with strontium chloride. Additionally, silanes may be added to improve properties of the coating. The coatings produced are mixed hydroxides and oxides.
The prior art approaches fail to address the need for environmentally sound coatings which can be used in a broad-spectrum of applications. Typically, coatings which have been developed to replace chromates do so in select applications, such as the use of silicates for unpainted air conditioning evaporators or silane treatments to enhance paint adhesion on substrates such as automotive air conditioning condensers.
There are many organic systems which can be used for improving corrosion resistance, but these typically involve use of solvents, rendering compositions which have VOC impact on the environment. It can be seen from the foregoing that the compositions of the prior art have not used Group IV-A metals combined with rare-earth metals in an aqueous, non-organic solvent system that excludes high levels of fluoride specifically. Additionally, the prior art does not show formation and attachment of Group IV-A gels, incorporating rare-earth elements, from acidic aqueous solution without using organic solvents. Sol-gels are macromolecular units rather than discrete atoms or molecular units and are typically prepared from metal-alkoxy precursors in solvent-based solutions that are unstable in water.
In addition, many health and environmental benefits of eliminating or reducing fluoride have been addressed in systems based on chemistries other than those of the Group IV-A metals used with Group III-A metals. Examples are described in UK Pat. Application 2,084,614 by Higgins.
In view of the foregoing, it can be seen that there exists a need for an improved "complete-spectrum" coating which can be used in a number of applications, and which is also environmentally sound and has a low impact in the workplace. This is currently a particularly strong need in aerospace and other applications where high-copper aluminum is used in large quantities due to its strength characteristics. The present invention provides such a coating.
Additionally, there is a need for compositions which render a surface highly resistant to corrosion and simultaneously provide a hydrophilic paint base. Hydrophilic Group IV-A/silicate coatings give excellent corrosion resistance but are generally unacceptable as a paint base. Hydrophobic, low fluoride Group IV-A coatings provide excellent corrosion resistance and paint adhesion, but they develop a hydrophobic surface which is detrimental to efficient heat exchange in applications such as for evaporators in automotive air conditioning units. They are ideal as paint bases for condensers in automotive air conditioning units, which are typically painted black for aesthetic and protective purposes.
Automotive air conditioning condensers and evaporators are typically produced in one plant. Therefore, a single coating for each is desirable to save on floor space and capital equipment costs. This is one need the present invention addresses as the coatings produced can have formulants balanced such that each criterion can be met to the degree individual manufacturers specify.
From the foregoing, it will be appreciated that there exists a need for broad-spectrum coating systems which are aqueous, promote paint adhesion and environmental resistance simultaneously. Additionally, it is desirable that such systems be balanced with regard to their hydrophilic to hydrophobic nature. In this way, systems can be designed to make available a "single-package" product for coating a number of products with differing performance requirements in a single facility. It is further desirous that the coatings impart superior corrosion protection to metal substrates.
It is an object of the present invention to provide such compositions, as well as processes for coating substrates that incorporate said compositions. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.